1. Field of the Invention
The present invention relates to a distributed feedback type semiconductor laser device and a method of manufacturing the same, and, more particularly, to a gain coupled distributed feedback type semiconductor laser device capable of reducing a variation in laser characteristics typified by the oscillation threshold current and luminous efficiency, and a method of manufacturing the same.
2. Prior Art
Distributed feedback type semiconductor laser devices have a predetermined layer structure comprised of semiconductor materials and a cavity of a predetermined cavity length in which a diffraction grating for periodically changing the real part or the imaginary part of the refractive index is formed. The semiconductor laser devices have such a wavelength selectability that the diffraction grating feeds back only a laser beam of a specific wavelength. In the distributed feedback type semiconductor laser devices, the diffraction grating that has the aforementioned capability is generally formed evenly over the entire area between both facets of the cavity in the lengthwise direction (the waveguide direction of light).
The distributed feedback type semiconductor laser devices include an index coupled type in which the real part of the refractive index periodically varies in the cavity, a gain coupled type in which the imaginary part of the refractive index periodically varies in the cavity, and a complex coupled type in which the real part and the imaginary part of the refractive index both periodically vary in the cavity.
The index coupled type semiconductor laser device frequently oscillates in two modes near the Bragg wavelength. When there is no reflection on both facets of the cavity, particularly, this semiconductor laser device always oscillates in two modes near the Bragg wavelength. Even when there is reflection on the facets of the cavity, the semiconductor laser device may oscillate in one of the two modes. This oscillation depends on the phase of the diffraction grating that is located in the vicinity of the facets of the cavity.
The facets of a semiconductor laser device are normally formed by cleaving. It is however difficult to control the phase of the diffraction grating on (and near) the facets of the cavity by cleaving. Therefore, the phase of the diffraction grating on the facets of the cavity and, eventually, the single-mode characteristic defined by that phase vary from one semiconductor laser device manufactured to another. In other words, the index coupled type suffers a lower single-mode yield.
Even when there is reflection on the facets of the cavity, the gain coupled type semiconductor laser device is more likely to have single-mode oscillation as compared with the index coupled type. The pure gain coupled distributed feedback type semiconductor laser device essentially oscillates at the Bragg wavelength in single mode.
According to the complex coupled type, one of two modes which sandwich the stop band in between is likely to be selected so that the single-mode yield becomes higher than that of the index coupled type.
To acquire a high single-mode yield, the gain coupled type distributed feedback type semiconductor laser device is more suitable than the index coupled type.
Though having less influence of reflection at the facets than the index coupled type, the gain coupled type is actually affected by such reflection however small it is. Actually, a variation in the laser characteristics of the gain coupled type is recognized depending on the phase of the diffraction grating on the facets of the cavity. The following explains the reason.
In the gain coupled distributed feedback type semiconductor laser device, while the crest of the standing wave in oscillation mode which is formed by the distributed feedback is coupled to that part of the diffraction grating which has a large gain, the trough of the standing wave is coupled to that part of the diffraction grating which has a small gain. Because the facets of the cavity of a semiconductor laser device has free end reflection, the crest of the standing wave formed by the facet reflection always coincides with the positions of the facets of the cavity.
Since, as mentioned above, it is difficult to control the positional relationship between the facets (cleaved faces) of the cavity and the diffraction grating depending on cleaving at the time of manufacturing the semiconductor laser device, however, the crest and trough of the standing wave formed by the distributed feedback do not necessarily coincide with the crest and trough of the standing wave formed by reflection at the facets of the cavity and they vary from one manufactured device to another.
When the crest and trough of the standing wave formed by the distributed feedback match with those of the standing wave formed by the reflection at the facets of the cavity, the acquired semiconductor laser device has a low oscillation threshold current and high luminous efficiency. When those two standing waves do not have matched crests and troughs, however, the acquired semiconductor laser device may oscillate in single mode but suffers degraded laser characteristics, such as an increased oscillation threshold current and reduced luminous efficiency.
As apparent from the above, the gain coupled distributed feedback type semiconductor laser device that has the diffraction grating formed over the entire area in the cavity has the inherent problem of causing a variation in laser characteristics as the standing wave formed by the distributed feedback and the standing wave formed by the reflection at the facets of the cavity enhance or weaken each other based on a variation in the phase of the diffraction grating on the facets of the cavity which is caused by cleaving at the time of manufacturing the semiconductor laser device.
Accordingly, it is an object of the present invention to provide a gain coupled distributed feedback type semiconductor laser device which has a high single-mode yield and a smaller variation in laser characteristics typified by the oscillation threshold current and luminous efficiency, and a method of manufacturing the same.
The present inventors contemplated on the following points while they made extensive studies to achieve the above object.
(1) First, the crest of the standing wave that is formed by the distributed feedback is gain-coupled to that part of the diffraction grating which has a large gain, and the trough of the standing wave is coupled to that part of the diffraction grating which has a small gain.
(2) By contrast, the crest of the standing wave that is formed by reflection of a laser beam which is caused by other than distributed feedback, specifically, reflection on the facets of the cavity, is always positioned on those facets of the cavity.
(3) Because the phase of the diffraction grating on the facets of the cavity that are formed by cleaving varies, the facets of the cavity may not be the large-gain portion of the diffraction grating. Therefore, the crest of the standing wave that is formed by the distributed feedback need not be surely coupled to the large-gain portion of the diffraction grating formed on (and near) the facets of the cavity. This causes various interferences between the distributed feedback and the reflection on the facets of the cavity, resulting in a variation in laser characteristics.
(4) If the diffraction grating having such a structure as to permit the crest of the standing wave formed by the distributed feedback to be surely coupled to the large-gain-coupling portion of the diffraction grating at (and in the vicinity of) the facets of the cavity is formed inside the cavity, therefore, it is possible to suppress the aforementioned interferences between the distributed feedback and the facets of the cavity.
(5) The above structure can be achieved by forming the diffraction grating over the entire area in the diffraction grating in such a way that the gain coupling coefficient of that portion of the diffraction grating which is located at (and in the vicinity of) the facets of the cavity or by not forming the diffraction grating at (and in the vicinity of) the facets of the cavity, so that the gain coupling does not occur there.
The present inventors conducted experiments in consideration of those points, and having confirmed that their contemplation was correct, developed a gain coupled distributed feedback type semiconductor laser device of this invention.
According to the first aspect of this invention, there is provided a gain coupled distributed feedback type semiconductor laser device whose gain or loss periodically changes, which comprises a cavity: and a diffraction grating formed in the cavity in such a way that an absolute value of a gain coupling coefficient in a vicinity of an area where reflection of a laser beam is caused by other than distributed feedback is smaller than an absolute value of a gain coupling coefficient in the other area.
According to the second aspect of this invention, there is provided a gain coupled distributed feedback type semiconductor laser device whose gain or loss periodically changes, which comprises a cavity; and a diffraction grating formed in the cavity in such a way that an absolute value of a first gain coupling coefficient in a vicinity of at least one of facets of the cavity is smaller than an absolute value of a second gain coupling coefficient in the other area. In this case, the length of the at least one of the facets of the cavity where the absolute value of the first gain coupling coefficient is smaller than the absolute value of the second gain coupling coefficient may be at least 10 xcexcm, and especially, the diffraction grating may not be formed at least in a vicinity of the facets of the cavity.
According to the third aspect of this invention, there is provided a method of manufacturing the distributed feedback type semiconductor laser device of the first aspect or the second aspect, which is characterized in that an electron beam writing system is used at a time of forming a diffraction grating in a cavity.
According to the fourth aspect of this invention, there is provided a method of manufacturing the distributed feedback type semiconductor laser device of the first aspect or the second aspect, which is characterized in that at the time of forming a diffraction grating in a cavity, an interference exposure apparatus is used to form the diffraction grating over an entire area inside the cavity, and a part of the diffraction grating is then removed.